INTERACTION ANALYSES OF GENES REQUIRED FOR RESISTANCE RESPONSES TO POWDERY MILDEW IN BARLEY REVEAL DISTINCT PATHWAYS LEADING TO LEAF CELL-DEATH

Race-specific resistance in barley to the powdery mildew fungus (Erysi phe graminis f sp hordei) is associated with a cell death reaction (hy persensitive response [HR]). Genetically, it is dependent on dominant resistance genes (Mix), and in most cases, it is also dependent on Rar 1 and Rar2 Non-race-specific resistance to the fungus, which is due to the lack of the Mlo wild-type allele, is dependent on Ror1 and Ror2 a nd is not associated with an HR in the region of pathogen attack. Howe ver, the absence of the Mlo wild-type allele stimulates a spontaneous cell death response in foliar tissue. This response is also controlled by Ror1 and Ror2, as indicated by trypan blue staining patterns, Lack of Mlo enhances transcript accumulation of pathogenesis-related genes upon fungal challenge, and this response is diminished by mutations i n Ror genes, Using DNA marker-assisted selection of genotypes, we prov ide evidence, via gene interaction studies, that Ror1 and Ror2 are not essential components of race-specific resistance and do not compromis e hypersensitive cell death. Reciprocal experiments show that neither is Rar1 a component of mlo-controlled resistance nor does it affect sp ontaneous cell death, We show that mlo-and Ror-dependent resistance is active when challenged with E. g. f sp tritici, a nonhost pathogen of barley. Our observations suggest separate genetic pathways operating in race-specific and non-race-specific resistance; they indicate also a separate genetic control of hypersensitive and spontaneous cell deat h in foliar tissue.